Controllable integrators may be used for the realization of analogue filter circuits in bipolar semiconductor technology. They are suitable for the low and medium frequency range. Controllable integrators may be manufactured by means of conventional, standard processes of bipolar technology. The dependency on temperature of the filter characteristic may be compensated by the controllability of the integrator, and advantageous applications are possible with adjustable or controlled filters. For various reasons, filter circuits without inductors comprising identical, integrating amplifiers which are connected to one another by chain connection and summing elements are preferred. Such filter circuits exhibit a high repetition part degree, are highly flexible in design, and may be rendered low-noise and relatively insensitive of tolerances by means of suitable design methods.
It is well known that the characteristics of discretely designed filters and filter circuits in important points cannot be attained by methods of integration technologies. Suitable inductors cannot be manufactured and may only be substituted by the methods of the active RC circuits. The available capacitances are small and of low quality. Diffused or implanted resistors are temperature-dependent and clearly non-linear. Technological difficulties rapidly increase at values above a few 10s of kiloohms. Under these circumstances, special measures must be taken if selective circuits with border frequencies or resonances below approximately 20 kilohertz are to be realized.